Setting output power levels of transmitters, radio base stations and mobile stations or terminals in uplink is commonly referred to as power control in mobile systems. The main objectives of power control include improved capacity, coverage, user quality (bit rate or voice quality) and reduced power consumption. Power control mechanisms can be categorized into several groups: (i) open-loop power control, (ii) closed loop power control and (iii) combined open- and closed loop power control. These differ in what input is used to determine the transmit power. In the open-loop power control case, the transmitter measures a signal to noise ratio or a signal to interference ratio of a signal sent from the receiver, and the transmitter sets its output power based on this measured signal. The closed-loop power control case (also known as inner-loop power control) is the ability of the transmitter to adjust its output power in accordance with one or more transmit power control (TPC) commands received from the receiver, in order to keep the received signal to interference ratio (SIR) at a given SIR target. In the closed-loop power control, the receiver thus measures the SIR of a signal from the transmitter and then sends a TPC command to the transmitter. The transmitter then adapts its output power based on the received TPC command. In the combined open- and closed-loop power control case, both inputs are used to set the transmit power.
Power control mechanisms discussed above are widely used in wireless systems. In e.g. systems with multiple channels between mobile terminals and radio base stations, different power control principles may be applied to the different channels. An advantage of using different power control principles to different channels yields more freedom in adapting the power control principle to the needs of individual channels. A drawback is an increase in complexity of maintaining several power control principles to the needs of individual channels. As an example, if common TPC commands for closed-loop and combined open-loop and closed-loop power principles are used, multiple channels can use the same TPC command. A drawback with such solution is that it yields limited flexibility in adjusting power to the radio conditions of the individual channels. If instead separate TPC commands are used, the overhead is increased.